1. Field of the Invention
The present invention relates to a polarization maintaining optical fiber capable of transmitting optical signals while maintaining their polarization state, which can be utilized as a transmission medium in optical communication networks and optical signal processings.
2. Description of the Background Art
FIG. 1 shows a conventional optical fiber 10 in which a core 11 in a form of a hollow hole is surrounded by a photonic crystal structure cladding 12, and this photonic crystal structure cladding 12 is further covered by a jacket 13. Note that, in the literature, a term xe2x80x9cphotonic crystal structure claddingxe2x80x9d is used regardless of a material used for the core while a term xe2x80x9cphotonic band gap claddingxe2x80x9d is used specifically in the case of using a hollow hole core or the core with the refractive index lower than that of the cladding, and a term xe2x80x9cphotonic crystal structure claddingxe2x80x9d will be used throughout this specification in the former sense so that the xe2x80x9cphotonic crystal structure claddingxe2x80x9d should be construed as including the xe2x80x9cphotonic band gap claddingxe2x80x9d.
The photonic crystal structure cladding 12 has a diffraction grating (represented by blank dots in the figure), which is usually formed by hollow holes but it can also be formed by a material with a different refractive index in circular cross sectional shapes.
Next, the principle of optical waveguiding by the optical fiber 10 in such a configuration will be described. In this optical fiber 10, in the case where the material of the core 11 is glass, the equivalent refractive index of the photonic crystal structure cladding 12 is lower than that of the core 11, so that lights are waveguided within the core 11 by the confinement due to the total reflection of the refractive index (similar to the confinement in the general single mode fiber).
On the other hand, in the case where the refractive index of the core 11 is lower than that of the photonic crystal structure cladding 12 or in the case where the core 11 is a hollow hole so that its refractive index is equal to that of the air which is approximately 1, the photonic crystal structure cladding 12 and the jacket 13 in the surrounding are formed by silica materials same as those used in the ordinary optical fiber, so that their refractive indexes are higher than that of the core (hollow hole) 11. Consequently, if the cladding in the same structure as that of the conventional optical fiber is used, the refractive index of the hollow hole core 11 would become lowest and therefore it would be impossible to confine the energy of lights within the core 11 in this structure.
For this reason, the confinement of lights is realized by adopting a structure called photonic crystal structure in part of the cladding. Namely, the photonic crystal structure cladding 12 having a diffraction grating with an appropriate lattice interval for confining lights within the core 11 is provided in the surrounding of the core (hollow hole) 11.
FIG. 2 shows a configuration of the photonic crystal structure cladding 12. In general, the three-dimensional photonic crystal structure is a diffraction grating capable of causing the Bragg reflection of lights in all directions, which is realized by setting the lattice constant (lattice interval) d of the diffraction grating d approximately equal to the wavelength of lights to be propagated through the medium (core=air), as shown in FIG. 2.
There are many possible configurations for a crystal lattice constituting the photonic crystal structure besides a square lattice shown in FIG. 2. FIG. 3 shows some exemplary configurations for a crystal lattice constituting the photonic crystal structure.
FIG. 3A shows a square shaped lattice structure (black portions in the figure) with a higher refractive index which is embedded in a medium (white portion in the figure) with a lower refractive index. FIG. 3B shows a square shaped lattice structure (white portions in the figure) with a lower refractive index which is embedded in a medium (black portion in the figure) with a higher refractive index. FIG. 3C shows a triangular shaped lattice structure (black portions in the figure) with a higher refractive index which is embedded in a medium (white portion in the figure) with a lower refractive index. FIG. 3D shows a triangular shaped lattice structure (white portions in the figure) with a lower refractive index which is embedded in a medium (black portion in the figure) with a higher refractive index. FIG. 3E shows a honeycomb shaped lattice structure (black portions in the figure) with a higher refractive index which is embedded in a medium (white portion in the figure) with a lower refractive index.
According to J. D. Joannopoulos et al., xe2x80x9cPhotonic Crystalsxe2x80x9d, Princeton University Press, pp. 122-126, 1995, it is known that the photonic crystal structure is present in these configurations so that the confinement of lights can be realized.
Note also that the lattice structure with cylindrical or circular hole shaped lattice holes is assumed here, but the shape of the lattice holes is not necessarily limited to the cylindrical or circular hole shape, and the photonic crystal structure can also be realized by using the lattice structure with the lattice holes in a triangular prism or triangular hole shape, a rectangular bar or rectangular hole shape, a hexagonal bar or hexagonal hole shape, etc.
When the core in a form of a hollow hole is provided in this photonic crystal structure, lights are strongly confined within this core. Consequently, when it is desired to waveguide lights through some structure, it is possible to propagate lights while confining them within that structure (the core 11) by providing the photonic crystal structure (the photonic crystal structure cladding 12) in the surrounding of that structure (the core 11), as shown in FIG. 2.
This photonic crystal structure is provided in the surrounding of the optical fiber core to realize the confinement such that lights do not propagate in a radial direction from a center of the optical fiber core. Namely, as shown in FIG. 1, the cross section of the optical fiber 10 has a lattice shaped structure, and this same structure is maintained along the length direction. In other words, the cross section of the optical fiber 10 has a uniform structure throughout (except for a fluctuation of shape due to the fabrication process of the optical fiber) and there is no structure that is perpendicular or oblique to the length direction of the optical fiber 10. By adopting this structure, it is possible to propagate lights entered into the core 11 while confining them within the core 11.
In the conventional optical fiber 10 described above, although it is possible to transmit lights through the optical fiber while confining lights within the core 11, the following problem arises in the case of carrying out optical communications using this optical fiber 10. Namely, in the case where the shape of the core 11 of the optical fiber is circular, there is no mechanism for determining a polarization direction of lights propagating within the core 11, so that a fluctuation in the polarization direction within the core 11 can be caused by a slight fluctuation in this circular share of the core 11. Consequently, the polarization state of optical signals after transmission through the optical fiber 10 can vary due to causes such as temperature variation or vibration of the polarization maintaining optical fiber 10, and it has been necessary for a receiving side to use a polarization independent structure that is not affected by the variation of the polarization of optical signals.
The currently existing polarization maintaining optical fibers include the PANDA fiber which does not use the photonic crystal structure. The fabrication process of this PANDA fiber requires a sophisticated technique of forming holes at two locations in a vicinity of the core made by the material of the optical fiber and creating fibers by squeezing stress applying material into these holes. In particular, the process for squeezing the stress applying material is the major factor for preventing the improvement of the productivity of the PANDA fiber, and the price of the PANDA fiber is as high as 100 times or more of that of the ordinary single mode fiber. In addition, this PANDA fiber structure cannot realize a very large propagation constant difference between the orthogonal polarization modes, so that it has been difficult to realize the cross-talk of xe2x88x9230 dB or more between these two modes. For this reason, it is difficult to realize a long distance transmission of signal pulses while maintaining a single polarization by using the PANDA fiber, and the use of the PANDA fiber as a single polarization transmission path has not been realized so far.
It is therefore an object of the present invention to provide a polarization maintaining optical fiber capable of transmitting optical signals while maintaining the polarization of optical signals stably.
According to one aspect of the present invention there is provided a polarization maintaining optical fiber, comprising: a core in a cross sectional shape having different diameters along two orthogonal axes defined on a plane perpendicular to an optical axis; and a photonic crystal structure cladding which is provided in a surrounding of the core and having a diffraction grating with a lattice interval for realizing confinement of lights within the core.
According to another aspect of the present invention there is provided a polarization maintaining optical fiber, comprising: a core in a form of a hollow hole; and a photonic crystal structure cladding provided in a surrounding of the core and having a diffraction grating with lattice intervals for realizing confinement of lights within the core, the lattice intervals being different along two orthogonal axes defined on a plane perpendicular to an optical axis.
According to another aspect of the present invention there is provided a polarization maintaining optical fiber, comprising: a core; and a photonic crystal structure cladding provided in a surrounding of the core and having a diffraction grating with lattice intervals for realizing confinement of lights within the core, the lattice intervals being different along two orthogonal axes defined on a plane perpendicular to an optical axis; wherein the core is formed by a material having a lower refractive index than that of a material forming the photonic crystal structure cladding.
According to another aspect of the present invention there is provided a polarization maintaining optical fiber, comprising: a core; and a photonic crystal structure cladding provided in a surrounding of the core and having a diffraction grating with a lattice interval for realizing confinement of lights within the core, the photonic crystal structure cladding being divided into four divided portions along a circumferential direction, at least a part of lattice holes in a first pair of divided portions that are facing each other along one direction having a larger lattice hole diameter than a smaller lattice hole diameter of lattice holes in a second pair of divided portions that are facing each other along another direction orthogonal to the one direction, and a ratio of the larger lattice hole diameter and the smaller lattice hole diameter satisfies a condition for realizing a single peak in a light intensity distribution in a vicinity of the core.
According to another aspect of the present invention there is provided a polarization maintaining optical fiber, comprising: a core in a form of a hollow hole; and a photonic crystal structure cladding provided in a surrounding of the core and having a diffraction grating with a lattice interval for realizing confinement of lights within the core, the photonic crystal structure cladding being divided into four divided portions along a circumferential direction, and at least a part of lattice holes in a first pair of divided portions that are facing each other along one direction having a larger lattice hole diameter than a smaller lattice hole diameter of lattice holes in a second pair of divided portions that are facing each other along another direction orthogonal to the one direction.
According to another aspect of the present invention there is provided a polarization maintaining optical fiber, comprising: a core; and a photonic crystal structure cladding provided in a surrounding of the core and having a diffraction grating with a lattice interval for realizing confinement of lights within the core, the photonic crystal structure cladding being divided into four divided portions along a circumferential direction, and at least a part of lattice holes in a first pair of divided portions that are facing each other along one direction having a larger lattice hole diameter than a smaller lattice hole diameter of lattice holes in a second pair of divided portions that are facing each other along another direction orthogonal to the one direction; wherein the core is formed by a material having a lower refractive index than that of a material forming the photonic crystal structure cladding.
Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.